Medical injection systems such as pen injectors are supposed to allow patients to self-administer required doses of medicaments, such as insulin. When injecting medicaments, dose accuracy is of therapeutic relevance. Conventional injection systems comprise a carpule, e.g. filled with insulin. A stopper is arranged in the carpule in a manner to be translated by a dosing mechanism of the pen injector by a distance corresponding to the required dose thus displacing the dose from the carpule through an injection needle into the injection site, e.g. the patient's skin. Dose accuracy in such systems depends on the precision of the translation of the stopper, tolerances in the internal diameter of the carpule and the compressibility of the stopper.
WO 2006/032070 A1 discloses a device for the dosed intake and delivery of a liquid, especially an infusion liquid. Said device comprises a cylinder embodying a liquid chamber that can be pressurised, a piston which is guided in the cylinder along the cylinder axis, and at least one line for introducing liquid into the liquid chamber and for dispensing liquid from the same. The aim of the invention is to ensure that the device is easy to handle and to prevent the risk of contamination of the infusion solution as far as possible. To this end, the piston can be displaced in the cylinder, against an elastic force, from an idle position defined by an abutment, to an end position defined by an abutment, in order to displace a defined cylinder volume.